Pages

Sunday, April 29, 2018

A Comet or Titan: The Next New Frontiers Mission

The two
concepts contending to be selected as NASA’s next planetary mission have two
things in common.

Both
would do compelling science.

Both
would not begin their scientific missions until the mid-2030s.

Otherwise
the two missions could not be more different.

The Comet Astrobiology Exploration
Sample Return (CAESAR) mission would return to the comet 67P/Churyumov-Gerasimenko
that the Rosetta spacecraft and its lander Philae previously studied.The CAESAR spacecraft essentially would be a high-tech
truck that would grab a small sample from the surface and then return it to
Earth.Except for some ancillary observations
performed by its engineering cameras, the science would begin when the samples
are delivered to the exquisitely sophisticated instruments in terrestrial
laboratories where they would be analyzed grain-by-grain and
isotope-by-isotope.

The
Dragonfly mission would return to the surface of Saturn’s moon Titan that is at
once both an Earth-like and an utterly alien world.The craft would alternate between flights to
new locations and much longer periods as a stationary surface station. Every flight would be a voyage of discovery
above a barely known surface.Every
landing could provide a new site to explore in depth.Every minute of its potentially many year
sojourn at Titan would be spent doing science.

Both
missions are finalists in the competition to be NASA’s fourth New Frontiers
mission.(Previous selections were the
New Horizon’s Pluto-Kuiper belt spacecraft, the Juno Jupiter orbiter, and the
OSIRIS-REx asteroid sample return mission.)These are the mid-class missions of NASA’s solar system exploration
program that are launched once to twice a decade.NASA’s managers selected these two concepts
from a field
of twelve proposals (and here).

The
CAESAR and Dragonfly missions differ so much because they represent different stages
in the progression of exploration.Comets have already had several flyby missions and one orbiter-lander
mission.As a result, the top priority
among comet specialists is to progress to the next step, sample return.Further exploration isn’t the priority. The lowest risk mission that grabs and brings
back the sample is the best mission.

Titan, on
the other hand, was studied by the Cassini spacecraft through a multitude of
flybys that cumulatively mapped the surface at only low to moderate resolution.(Titan’s atmosphere was also explored by the
Huygens probe that descended to the surface, but it carried out only the
simplest measurements of surface’s physical properties once it landed.)The next step is to land to study the surface
in detail.Titan offers a unique
opportunity in the solar system where a thick atmosphere coupled with low
gravity allows a science station to relocate itself by flying to new
sites.For this world, exploration is
the priority.

CAESAR

Comets
are deep frozen remnants from the birth of our solar system.They contain relatively unaltered ices and
dust from the interstellar cloud from which our solar system formed as well as
the processed material that resulted from the earliest steps in the formation
of comet-sized bodies.By returning
samples to Earth, scientists can determine what materials were available to
build the planets.They also can study
the earliest chemical pathways that led to creating more complex molecules and
the physical processes that accumulated the interstellar material into larger
bodies.As one recent scientific paper
put it, “comets record chemical evolution in the protoplanetary disk and allow
us to unveil the formation history of the organics and volatiles,” while
another in the same issue notes that the Rosetta spacecraft found that comet
67P, “revealed a greater variety of molecules than previously identified and indicated
that 67P contained both primitive and processed organic entities.”

The CAESAR mission would return to comet
67P/Churyumov-Gerasimenko.Credit:
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA;
processing by Giuseppe Conzo

However, instruments
that can fit the tight mass, size, and power budgets to be flown on spacecraft are
limited in their ability to explore the composition of these materials.The far more sophisticated instruments in
Earth-based laboratories – which can be larger than entire spacecraft or even
launch vehicles – can study samples grain-by-grain and isotope-by-isotope.The ultimate exploration goal, where
technically feasible, for each world is to bring samples back to our world to
explore their formation and evolution in depth.

The team proposing the CAESAR mission
has laid out a series of hypotheses that would be enabled by examining samples
in terrestrial laboratories.Arranged in
three themes, the hypotheses address different periods in the formation of the
solar system starting with the transition from the interstellar cloud from
which the solar system formed.The
hypotheses for this theme address questions on what types of unaltered interstellar
material were preserved (or not) in 67P.The next theme addresses the nature of the material in the
protoplanetary disk and hence the material that would have been available to
build the earliest small bodies.Here
the key questions focus on whether 67P is a direct descendant of the
interstellar medium or also contains high temperature silicates and organics
that would have formed close to the early sun and then been blown out to the
outer solar system and trapped within the material that formed 67P.The final theme addresses how the current
body that is 67P formed from the initial grains of ice and rock available in
the earliest solar system, and what role bodies like it played in delivering
materials, especially volatiles such as water, to the planets.

The
CAESAR mission would be a companion mission to the previously selected, and now
in flight, OSIRIS-REx New Frontiers mission.While comets are the oldest frozen remains of the solar system’s
formation, several classes of asteroids are considered primitive relics of the
earliest formation of solid bodies closer to the sun.The OSIRIS-REx spacecraft will arrive at one
of these, 101955 Bennu, later this year.Following a thorough examination of this tiny (~245-meter diameter) body
by a battery of remote sensing instruments, the spacecraft will descend, and a
robotic arm will grab a sample from the surface to be returned to Earth.

Collecting
samples from very small bodies is an engineering challenge because the gravity
is too low to enable conventional landings and surface operations.Like the Japanese Hayabusa and OSIRIS-REx
asteroid sample return missions, the CAESAR spacecraft would use a touch-and-go
sampling mechanism.The spacecraft would
slowly descend to the surface with a sample collection system at the end of a
long arm below the craft.During the
five seconds that the collection system would be in contact with the surface,
nitrogen gas would be used to blow the comet’s surface material into a
containment system.Because comets
contain ices that can freeze into a solid surface, the CAESAR collection system
(unlike the OSIRIS-REx system where ices aren’t expected) would have spring
loaded ripper tines to break up the surface.The mission’s goal is to collect at least 80 grams of dust and ices,
although the collection system can gather several times that if enough loose
material is available.After that brief
contact, the arm acts as a pogo stick pushing the spacecraft away from the
surface and back into space.

The CAESAR spacecraft with its extended arm and sampling
mechanism.Credit NASA.

Again, like
OSIRIS-REx, following confirmation from instruments that a sample was
successfully collected, the containment system with its sample would be placed
within a sample return capsule that ultimately would deliver the sample to
Earth following a fiery plunge through the atmosphere.However, this capsule and its embedded sample
containment system have two major innovations not needed by the OSIRIS-REx
mission.

For the
CAESAR mission, a key goal is to return both the dusty component and minimally
modified samples of the icy volatile material.Once in the sample return capsule, the sample would be gently heated to
approximately -30 degrees Celsius to sublimate the volatiles out of the
sample.(If -30 sounds chilly, remember
that comets are deep frozen relics even following heating by the sun during
their passages through the inner solar system.)The sublimated gases would flow to a gas containment system that would
be kept at -60 degrees Celsius where they would either refreeze (for example,
water) or remain as gases (for example, methane and carbon dioxide) depending
on their freezing points.Removing the
volatiles from the sample ensures that they don’t chemically modify the solid
sample during the return flight to Earth.Keeping them chilled in the gas containment system also minimizes
chemical interactions among the volatiles.

The need
to preserve a frigid sample system drives a second difference from the
OSIRIS-REx return system.During the
plunge through the Earth’s atmosphere at the end of the mission, the heat
shield would absorb considerable heat that would eventually raise the
temperature of the sample.For the rocky
material that the OSIRIS-REx mission would return, this isn’t a concern.However, the CAESAR mission would drop the
heat shield from the return capsule as soon as the parachute deploys,
minimizing any heating of the sample.(This is a technology developed by the Japanese for their Hayabusa
asteroid sample return missions, and their space agency would provide the
CAESAR return capsule.)

There’s
another important difference between the OSIRIS-REx and CAESAR missions.To intelligently select and later interpret
samples, scientists need to understand the parent body: What is its shape and
interior structure?What processes have
shaped it surface? How does the composition vary across the body?The OSIRIS-REx mission is going to a body
that has never been visited before.As a
result, that spacecraft carries a rich payload of cameras and spectrometers to
map the physical surface and its composition.If the mission only completes its survey of the asteroid Bennu but for
some reason fails to return its sample to Earth, it would have told us a great
deal about how this tiny primitive asteroid came to be.

The
CAESAR mission’s target, comet 67P, was previously studied by the Rosetta
mission and its partially successful Philae lander.As a result, the CASEAR spacecraft wouldn’t
need to carry its own scientific instruments.It does carry narrow- and wide-angle engineering cameras that would be
used to remap 67P’s surface, which is likely to have changed during its
subsequent perihelion passages from solar heating.(Identifying the changes would be a useful
scientific byproduct.)From these
images, the science and engineering teams would map the surface in terms of how
safely the spacecraft can approach the surface, whether the surface material
has physical characteristics compatible with the sampling mechanism, whether
the arm can deliver the sample mechanism to the surface, and the intrinsic
science value.

Based on
maps derived from the Rosetta images, the CAESAR team is currently expecting to
take its sample from one of the smooth terrains that appear to consist of
material that has slumped there from the rugged terrains that would be unsafe
to sample directly.The smooth terrains also
likely contain material delivered from the jets that spew a cloud of volatiles
and dust in the space around 67P each time it comes close to the sun.Because the rocky and organic materials
appear to be nearly uniformly distributed across surface, the key deciding
factor on the final sample location likely would be made based on the richness
of the volatiles present.

Once the
CAESAR spacecraft acquires its sample, it would quickly move away from the
comet to wait for the start of its journey back to Earth.If all goes well, the sample would be
delivered to the Earth’s surface in the midmorning of November 20, 2038,
fourteen years after the mission’s launch.Following the recovery of the sample capsule, the returned gases, ices,
and solids would be carefully curated and preserved.The initial analyses on a portion of the
sample would be conducted using laboratory instruments that would be 20 years
more advanced than those available today.It’s expected that the samples would continue to be analyzed for decades
following their arrival on Earth as scientists develop more refined questions
and yet more advanced instruments.

Dragonfly

While
comet P67 is a well explored world, the surface of Saturn’s moon Titan largely remains
a blurry enigma. Thanks to the Cassini
orbiter’s many flybys of the moon, we have low- to moderate-resolution maps of
the surface that show seas of liquid methane and ethane, extensive dune fields,
mountains, a paucity of craters, and river systems.We similarly have low resolution maps of
surface composition differences that are frequently ambiguous as to the actual material
present.We know that sunlight
interacting with the atmosphere creates complex hydrocarbons and nitriles that
continuously rain down from the upper atmosphere to the surface.Cassini’s instruments have observed changes
in the surface following rains of liquid methane.There’s an interior water ocean that may be
in contact with the silicate core that could, like the interior oceans of
Europa and Enceladus, be an abode for life.

The Dragonfly rotorcraft shown in its initial landing in a
flat area between Titan’s equatorial dunes.Credit: NASA

What we
know about the surface becomes the foundation for the fundamental questions
that require a return visit.What processes have reshaped the surface,
erasing, except in a few terrains, the many craters that should have accumulated?Why hasn’t the rain of hydrocarbons buried
the surface over the age of the solar system, and where does new methane come
from to replace that lost to the formation of the hydrocarbons?Are there cryovolcanoes that bring liquid water
to the surface?What types of pre-biotic
chemistries occur on a world rich in hydrocarbons?And is there evidence of life, either that
formed on the surface from the interaction of water and the hydrocarbons or
carried up from the deep subsurface global ocean?Or is there life based on non-water
chemistries?

These
questions cannot be answered by landing at any single location on Titan.Fortunately, this moon’s thick atmosphere and
low gravity make this the easiest world in the solar system for flight.The team proposing the Dragonfly mission have
designed a rotorcraft lander that can fly several tens of kilometers in a
single flight to a new site.In an hour
or so of flying, the craft might travel further than the Opportunity rover has
crawled across the surface of Mars in fourteen years.Over the course of several years at Titan,
the craft could explore a variety of terrains hundreds of kilometers apart.

The
Dragonfly proposal is possible because of the recent advances in multirotor
technology to enable steady flight (think of all the quadcopter hobby drones as
inexpensive examples) and autonomous flight.Cassini’s maps of the surface enable scientists to identify interesting
locations to visit but are too low resolution (the smallest details are
hundreds of meters across) to be useful to for inflight navigation.On each flight, Dragonfly would be given a
target location to fly to.During the
flight, a combination an inertial measurement unit, radar, and optical
navigation would be used to track progress.Once at a landing area, a laser system (lidar for those familiar with
the technology) would map the site to find a safe touch down site.

On arrival
at Titan, the Dragonfly craft, safely enclosed in its entry capsule, would
directly enter the atmosphere.Once the
initial descent is completed, the Dragonfly rotorcraft would separate from the
capsule and guide itself to its first landing.Much of Titan’s equatorial regions are covered by dunes that crest at 50
to 200 meters separated by flat plains two to four kilometers wide.The craft would use its sensors to guide
itself to one of these interdune areas to enable that crucial safe first
landing.

The sands
themselves are known to be composed of frozen organic particles that likely are
material blown in from large areas of the moon’s surface.With its first landings in the dune sea, the
Dragonfly lander can explore how far organic chemistry has proceeded on this
hydrocarbon rich world.The interdune
plains may also contain exposed water ice that is the bedrock of Titan’s
surface.Sampling the primordial water
ice crust is a high priority to explore the chemistry present when Titan formed
and the surface and atmosphere may have been much different than they are
today.

Because
Titan’s surface is likely rugged at small scales, the Dragonfly team won’t send
the craft to land at an unseen location (except for that first landing).Instead, on its first flight following the
initial landing, the craft would be sent to scout for a next safe landing site
and then would return to the initial (and known to be safe) landing site.On the second flight, the Dragonfly craft would
fly past the second known safe landing site to scout a third, and then return
to the second site.In this way, the
mission would leapfrog from one known safe site to the next, using its sensors
to make up for the lack of high resolution orbital imagery.

Beyond
the dune seas, Titan has a rich variety of terrains to explore.Life on the Earth is based on organic
chemistry enabled by liquid water.The
surface of Titan is too cold for liquid water – the water ice is as hard as
rock – but impacts would melt the ice, mixing the hydrocarbons with liquid H2O
for as long as tens of thousands of years.Where liquid water and the organic material on the surface mixed, complex
pre-biotic chemistry likely occurred, making Titan a natural laboratory to
explore the possible origins of life. These
sites would provide natural laboratories to explore the resulting pre-biotic
chemistry and explore avenues for the origins of life.

Cassini’s
data hint at features that may be extensive cryovolcanic flows where water from
the interior ocean reached the surface.If
the liquid water originated in the deep interior ocean, sampling these areas
may allow the Dragonfly instruments to determine the composition of that ocean,
including searching for signs of any life that may exist there.

While
liquid water on the surface has likely been a rare event, liquid methane plays
the role on Titan that water does on Earth.The methane falls as rain, flows through river systems, and ends in
lakes and seas.Close to the equator
where the Dragonfly craft would roam, it appears that the seas have largely
dried up.The evaporation of the seas
could have concentrated chemical reactions, and the present dry-to-moist
surface would preserve the resulting material for Dragonfly’s instruments to
sample.Several areas near the equator
have been identified as possible dry sea beds.(The currently-filled methane
seas in the north polar region would be in darkness and out of sight of Earth
for telecommunication during the Dragonfly mission.Their exploration would need to wait for
another mission a decade or so later).

The
Dragonfly’s explorations would be tempered by the need to carefully manage its
power use.The craft would carry a
radioisotope power source that at Titan would supply a constant ~70 Watts of
electrical power.(And as importantly, it
provides plentiful supply of waste heat to keep the craft warm on the -180-degree
Celsius (-290-degree Fahrenheit) surface.)That supply is too low for the major power-hungry activities – flight
and returning data to Earth – but is enough to recharge a large battery.Operations would proceed at the pace of each
Titan day (around 16 Earth days).During
the night, the battery recharges, followed by a daytime flight and a series of
science activities and data downlinks.During
the long night when Earth is out of sight, the craft would power down many
systems to keep its meteorology and seismic instruments powered and permit
battery charging.(The Curiosity rover
on Mars is similarly power limited.Its
radioisotope power source charges a battery that powers driving and many operations.The difference is that days on Mars have a
similar length to our own.)

The
public information on how far the Dragonfly craft might go in a single flight
is somewhat vague – the design is not yet complete and there’s a tradeoff
between larger batteries and weight and volume.One article discusses an example battery that could allow up to 60
kilometers flight.It then states that
flights would be kept substantially shorter to provide a safety margin.If the average distance between landing sites
is 20 kilometers when the goal is to transit distances, then over a five Earth
year mission, the craft might fly 1500 to 2000 kilometers.Not all Titan days, though, are likely to be
spent with the goal of putting kilometers on the odometer.If the craft finds a cryovolcanic volcano or
flow, for example, it may spend many months doing short flights to different
locations within the flow.Some Titan
days may be spent doing aerial reconnaissance to map the feature.Other Titan days might be spent doing bunny
hops between features in a single landing area.

Examples of terrains within a region on Titan.The 5 µm bright areas have been interpreted
as dried sea beds.The distance across
the more detailed map is approximately 2700 kilometers at the equator.Credit: Dragonfly team.

Rather
than thinking of the Dragonfly craft as a global trekker, it seems more apt to
think of it as performing the equivalent of the missions of several rovers on
Mars.It would be able to transport
itself between and within several key landforms within a region of the
moon.Choose that region carefully,
though, and there’s a rich diversity to explore.Within the black box on the map above lie
seas of sand dunes, several features that may be cryovolcanic, possible dried
sea beds, and what may be one of the oldest exposed terrains on the surface in
the Xanadu highlands with its mountains, river channels, and possible impact
craters.

During
its flights, a set of cameras would look forward and downward during flights to
map the.The Huygens probe showed that
surface features can be distinguished in monochrome aerial images, and Dragonfly
would have some color imaging capability to discriminate differences in
composition.The mission’s operators,
though, would need to balance the number of images returned with other demands
on the mission’s power supply.The
strategy would be to return thumbnails of images, from which the mission team
would select a subset to be returned at higher resolution.

After
each landing, panoramic cameras would examine the local area while microscopic
cameras take close-up images.(Depending
on the wavelengths that the cameras can detect, Saturn may be visible in the
images in one of the spectral windows in which the atmosphere is
transparent.)Once on the surface, a
gamma-ray and neutron spectrometer would measure the bulk surface composition
to discriminate between, for example, pure water ice, ammonia-rich water ice,
and a layer of hydrocarbons.

Several
instruments would study meteorology both in flight and on the surface including
temperature, wind, and the methane humidity.Two instruments would listen for seismic activity and measure variations
in the extremely low frequency electromagnetic field to study the subsurface to
the depth of the interior water ocean.Other sensors would record the thermal and electrical properties of the
surface immediately around the landed craft.

The
mission’s star instrument, however, would be its mass spectrometer.This instrument, derived from similar
instruments for the Mars Curiosity and the European ExoMars rovers, would be
able to detect the presence of different ices and hydrocarbons at minute
levels.By measuring the precise
chemistry of ices from different locations, scientists can understand the
processes that created them.

This
instrument’s sensitivity would make Dragonfly an exciting astrobiology mission.It could, for example, measure tiny amounts
of key organic compounds such as amino acids, lipids, and sugars, as well as
the chirality of organic molecules.At
the minimum, Titan should have a treasure trove of pre-biotic material that
scientists would use to explore the chemical pathways leading to the complexity
of life.And Titan is a possible abode
for life, either as-we-know-it based on water in the deep ocean or novel forms
of life based on liquid methane.Proving
that life exists at the cellular level likely would be hard, but Dragonfly’s
measurements could be the bridge to future, even more sophisticated missions.

-----------------

Both CAESAR
and Dragonfly would be compelling missions.It is unfortunate that only one will be selected and the next
competition is likely to be several years in the future.

Launch
for the CEASAR mission would occur in the summer of 2024 and arrive at comet 67P
in December 2028.The spacecraft would
depart from the comet in late 2033 and return to Earth five years later.

Launch
for the Dragonfly mission would occur in 2025 with arrival at Titan in the mid-2030s.Absent a crash or equipment failure, the
length of the mission would be dictated by the slow decay of power delivered
from its power supply.Eventually it would
take two, then three, and then more Titan days to recharge the battery.At some point, power would be too low to
enable any flight.Then the craft would
become a stationary science station as power gradually fades until it becomes
silent likely sometime in the late 2030s or 2040s.

The
decision between these two excellent but fundamentally different missions will
be made in mid-2019 by NASA’s Associate Administrator for Science.His decision will determine whether the
2030s brings a flowering of science for comets or Titan.

I would like to thank Steve Squyres, principal investigator for the CAESAR proposal, and Elizabeth (Zibi) Turtle and Jason Barnes, PI and deputy PI for the Dragonfly proposal, for their comments on an earlier draft of this story.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.